1. Field of the Invention
The present invention relates to a method of manufacturing a photovoltaic device, and more particularly, to a method of manufacturing a photovoltaic device used in a solar cell, the photovoltaic device including a fine crystalline semiconductor layer.
2. Description of the Related Art
A solar cell is a type of photovoltaic device that converts solar energy directly into electricity. It is a key element in solar power generation.
The most basic structure of a solar cell is a P-N junction diode. Solar cells may be classified into compound solar cells using CIGS(CuInGaSe2) or CdTe, Group III-V solar cells, dye-sensitized solar cells, organic solar cells, and silicon solar cells according to the material of a light-absorbing layer. Furthermore, silicon solar cells may be classified into crystalline solar cells and thin-film solar cells.
A thin-film solar cell includes thin films coated on a transparent substrate, such as thin glass or plastic, or a metal substrate such as stainless foil. Due to characteristics of thin films therein, thin-film solar cells have a far shorter diffusion length of carriers than crystalline solar cells. Thus, thin-film solar cells in a PN junction structure have very low collection efficiency of electron-hole pairs. For this reason, thin-film solar cells have a PIN structure in which an intrinsic semiconductor material having a high light absorption rate is inserted between a P-type semiconductor and an N-type semiconductor.
Light efficiency of a solar cell is affected by the structure and design of the solar cell and the quality and thickness of each thin film of the solar cell. In particular, the structure of a solar cell must be selected very carefully.
A thin-film solar cell using amorphous silicon (a-Si:H) and fine crystalline silicon (mc-Si:H) or amorphous silicon-germanium (a-SiGe:H) utilizes a thin film with a thickness of several microns or less as a light absorbing layer. In addition, silicon itself has a low light absorption coefficient. Thus, there is a limit to achieving high light efficiency using a single PIN junction. To address this problem, a layered solar cell, in which a PIN solar cell having amorphous silicon (a-Si:H) and fine crystalline silicon (mc-Si:H) is formed in double or triple layers, is used. In this layered solar cell, the PIN solar cells are connected in series to each other, thereby increasing an open-circuit voltage and improving the efficiency of converting incident light into electricity.
A fine crystalline semiconductor material advantageously has a high light absorption rate in a long wavelength region and low optical degradation characteristics. However, a fine crystalline semiconductor material is generally required to have a thickness of 2 to 3 microns and a crystallization of 60 to 70%. Thus, it takes a long time to form a fine crystalline semiconductor material using a conventional deposition method that has a very low deposition speed.
In this regard, a lot of researchers have suggested various equipments and technologies, such as hot-wall chemical vapor deposition (CVD), very high frequency CVD (VHF CVD), ion-coupled VHF CVD (ICP-VHF CVD), and microwave plasma enhanced CVD (PECVD), to increase the deposition speed of a fine crystalline semiconductor material. However, the improvement in the deposition speed is negligible.
Meanwhile, technologies for forming an amorphous semiconductor which can be formed relatively faster than a fine crystalline semiconductor and converting the amorphous semiconductor into a fine crystalline semiconductor or a polycrystalline semiconductor through a separate crystallization process are being variously applied in display devices, such as liquid crystal display substrates and organic light-emitting diodes. Of the above technologies, a solid phase crystallization (SPC) method has the advantage of achieving crystallization using inexpensive equipment. However, the SPC method requires a high crystallization temperature of 600 to 700, and a long processing time of 1 to 24 hours. Thus, a transparent substrate or a metal substrate having a low melting point cannot be used. Moreover, since a polycrystalline semiconductor material formed by the SPC method includes a lot of crystal lattice defects, such as twin crystals, and has a large grain size, its light absorption rate is reduced.